1. Field of the Invention
The present invention relates to base stocks including heavy mineral oil base stocks, Gas-to-Liquids (GTL), hydrodewaxed, and hydroisomerized waxy feed base stocks and to such stocks of reduced/mitigated haze formation.
2. Related Art
Feed stocks for lubricating oil base stocks are generally mixtures of various carbon number hydrocarbons including by way of example and not limitation various carbon chain length paraffins, iso-paraffins, naphthenes, aromatics, etc. The presence of long carbon chain length paraffins in the hydrocarbon base stock causes pour point and cloud point to be relatively high, that is, the onset of solid wax formation in the oil occurs at relatively high temperature.
For lubricating oils to effectively function in their intended environments (internal combustion engines, turbines, hydraulic lines, etc.) they must remain liquid at low temperatures.
To this end hydrocarbon feed stocks used for lubricating oil base stock production are subjected to wax removal processes including solvent dewaxing wherein the wax is physically removed from the oil as a solid at low temperature using a solvent, or catalytic dewaxing using a catalyst that converts long chain normal or slightly branched long chain hydrocarbon (wax) by cracking/fragmentation into shorter chain hydrocarbon, to thereby reduce pour point and cloud point (both of which are measured at low temperature).
Waxy hydrocarbon feeds, including those synthesized from gaseous components such as CO and H2, especially Fischer-Tropsch waxes are also suitable for conversion/treatment into lubricating base oils by subjecting such waxy feeds to hydrodewaxing or hydroisomerization/cat (and/or solvent) dewaxing whereby the long chain normal-paraffins and slightly branched paraffins are rearranged/isomerized into more heavily branched iso-paraffins of increased viscosity index and reduced pour and cloud point. Lubricating oils produced by the conversion/treatment of waxes produced from gaseous components are known as Gas-to-Liquids (GTL) base oils/base stocks.
Despite being of reduced low temperature pour point and cloud point, however, heavy base stocks including heavy mineral oil base stocks and heavy GTL base stocks are also subject to low level haze formation which appears at temperatures usually higher than those traditionally used to measure pour point or cloud point. The onset of haze is seen on standing at ambient temperatures, e.g., room temperature, i.e. temperatures between about 15 to 30° C., more usually 20 to 25° C.
The haze precursors are wax types which are more difficult to remove than are the waxes typically associated with pour point and cloud point and do not necessarily respond to conventional wax removal techniques such as solvent or catalytic dewaxing or would do so only with severe loss in yield.
Dewaxing using diluent components such as MEK, MIBK, and mixtures with toluene at low temperatures followed by filtration using cloth media are well known in the literature (see, for example, DILCHILL™ Exxon Mobil Corporation). These methods do not remove the small amounts of haze or haze precursors because the waxy particles are too small to be trapped on the filter cloth media used in such solvent dewaxing processes. In addition, those methods use considerable energy and are prohibitive to use for dehazing when not already in place for dewaxing. Also, imperfections in the filter cloth due to manufacturing flaws or wear in service can allow enough wax to leak through to cause haze to develop immediately or upon standing.
Methods based on adsorption of wax haze particles on fixed beds of pellets or powders have been described. They suffer from the inability to achieve acceptable combinations of adsorptive capacity, pressure drop across the adsorbent bed, and yield loss during the slow regeneration process required by such devices.
As previously indicated, haze can form in oils merely upon standing at room temperature even after the oil has been dewaxed to a low pour point such as −5° C. or even lower. Haze disappears on heating but can reappear on standing and even at room temperature. The waxes associated with haze are predominantly paraffinic in nature and include iso-paraffins and n-paraffins which are higher molecular in weight than are the waxes usually associated with pour point and cloud point.
Haze formation reduces the desirability of the oil for lubricating oil formulations from a visual perspective of quality.
A particularly challenging situation occurs when the haze does not form within about two days after manufacture, during which certification tests are made, but rather later after the lubricant base stock has been shipped to a lubricant blender or even after the lubricant product has been shipped to a lubricant user.
From a customer perspective, the appearance of haze has negative implications with regard to quality, customers usually associating high quality with oils exhibiting a clear and bright appearance on visual observation. The clear and bright standard is in accordance with ASTM D-4176-93 (Reapproved 1997). Haze can also be quantified under a turbidity test criterion expressed as nephelometric turbidity units (NTU) having a maximum value of 24. NTU is measured by a turbidimeter such as a Hach Model 18900 ratio turbidimeter, a Hach Model 2100P turbidimeter, etc. employed under the conditions specified by the manufacturer.
Other methods for determining turbidity include: ASTM D6181, Standard Test Method for Measurement of Turbidity in Mineral Insulating Oil of Petroleum Origni; ASTM D5180, Standard Test Method for Turbidity in Clear Liquids; ASTM D1889, Standard Test Method for Turbidity in Water.
Haze is also seen as posing a potential for problems during use insofar as the wax associated with the haze have the potential to clog the pores of the fine filters employed, for example, when using industrial circulating oils.
To address haze formation in hydroisomerized synthetic wax heavy lube oil having a kinematic viscosity @ 100° C. of about 10 mm2/s or greater mitigation steps such as higher reactor severity to create more isomerized product help lower the extent or intensity of haze but are generally, by themselves, insufficient, and also result in a reduced yield of the desired product. Restricting the distillation range to lower boiling molecular weights is also effective in reducing the haze potential of the oil but much of the 1000° F.+ range lube base stock will be sacrificed in that case.
Haze has been addressed in the recent art.
U.S. Pat. No. 6,579,441 reduces haze in lubricating oil base oil feeds by contacting the oil with a solid adsorbent to remove at least a portion of the haze precursors. The solid adsorbents reduce the cloud point and haze of the oil with minimal effect on yield. Sorbents used in the process are generally solid particulate matter having high sorptive capacity and with a surface having some acidic character. Acid character is determined by measurement of acid site density, determined using, e.g., infra-red spectroscopic measurement of adsorbed basic molecules such as ammonia, n-butyl amine or pyridine. Sorbent materials include crystalline molecular sieves, alumino-silicate zeolites, activated carbon, aluminas, silica-alumina, and clays (e.g., bauxite, Fullers Earth, attapulgite, montmorillonite, halloysite, sepiolite) in various forms, e.g., powder, particles, extrudates, etc.
The oil to be treated is contacted with the adsorbent in batch mode or under continuous conditions using a fixed bed, moving bed, slurry bed, simulated moving bed, magnetically stabilized fluidized bed employing upflow, downflow or radical flow oil circulation, at temperatures usually below 66° C. and more preferably between about 10° C. and 50° C.
See also U.S. Pat. No. 6,468,417 and U.S. Pat. No. 6,468,418.
WO 2004/033607 teaches heavy hydrocarbon compositions useful as heavy lubricant base stocks. The heavy hydrocarbon composition comprise at least 95 wt % paraffin molecules of which at least 90 wt % are iso-paraffins, having a KV by ASTM D-445 of above 8 mm2/s at 100° C., an initial boiling point of at least 454° C. and an end boiling point of at least 538° C. This heavy hydrocarbon composition of this application is a particular GTL heavy oil made from Fischer-Tropsch wax subjected to hydroisomerization. This heavy stock will typically be mildly hydrofinished and/or dehazed after hydrodewaxing to improve color, appearance and stability. It is stated that dehazing is typically achieved by either catalytic or absorptive methods to remove those constituents that result in haziness but no details are provided.
U.S. Pat. No. 6,699,385 teaches a process for producing a low haze heavy base oil including the steps of providing a heavy waxy feed stream having an initial boiling point greater than 900° F. and having a paraffin content of at least 80%, separating the heavy feed stream into a heavy fraction and a light fraction by deep cut distillation, and hydroisomerizing the light fraction to produce a low haze heavy base oil. In this patent “low haze” means a cloud point of 10° C. or less, preferably 5° C. or less, more preferably 0° C. or less. It does not appear to mean haze which forms on standing at room temperature.
WO 2005/063940 teaches a process for preparing a haze-free base oil having a cloud point of below 0° C. and a kinematic viscosity at 100° C. of greater than 10 mm2/s by hydroisomerization of a Fischer-Tropsch synthesis product, isolation of one or more fuel products and a distillation residue, reduction of the wax content of the residue by contacting the residue with a hydroisomerization catalyst under hydroisomerization conditions and solvent dewaxing the hydroisomerized residue to obtain a haze-free base oil. See also WO 2005/063941.
U.S. Pat. No. 6,962,651 teaches a method for producing a lubricant base oil comprising the steps of hydroisomerizing a feedstock over a medium pore size molecular sieve catalyst under hydroisomerization conditions to produce an isomerized product have a pour point of greater than a target pour point of the lubricant base oils, separating the isomerized product into at least a light lubricant base oil having a pour point less than or equal to the target pour point of the lubricant base oil and into a heavy fraction having a pour point of equal to or greater than the target pour point of the lubricant base oils and a cloud point greater than the target cloud point of the lubricant base oils and, dehazing the heavy fraction to proved a heavy lubricant base oil having a pour point less than or equal to the target pour point of the lubricant base oils and a cloud point less than or equal to the target cloud point of the lubricant base oils. The feedstock can be Fischer-Tropsch wax. Dehazing is described as a relatively mild process and can include solvent dewaxing, sorbent treatment such as clay treating, extraction, catalytic dehazing and the like.
U.S. Pat. No. 6,080,301 teaches a premium synthetic lubricating oil base stock having a high VI and a low pour point made by hydroisomerizing a Fischer-Tropsch synthesized waxy paraffinic feed wax and then dewaxing the hydroisomerate to form a 650-750° F.+ dewaxate. Fully formulated lube oils can be made from appropriate viscosity fractions of such base stock by addition of suitable additives which include one or more of a detergent, a dispersant, an antioxidant, an antiwear additive, a pour point depressant, a VI improver, a friction modifier, a demulsifier, an anti-foamant, a corrosion inhibitor and a seal swell control additive.
US Published Application 2005/0261147 teaches lubricant blends with low Brookfield viscosities, the base oil being a mixture of a base oil derived from highly paraffinic wax and a petroleum derived base oil and containing a pour point depressant. Representative of base oils derived from highly paraffinic wax are base oils derived from Fischer-Tropsch wax via hydroisomerization. Pour point depressants are described as materials known in the art and include, but are not limited to esters of maleic anhydride-styrene copolymers, polymethacrylates, polyacrylates, polyacrylamides, condensation products of haloparaffin waxes and aromatic compounds, vinyl carboxylate polymers, terpolymers of dialkyl fumarates, vinyl esters of fatty acids, ethylene-vinyl acetate copolymers, alkyl phenol formaldehyde condensation resins, alkyl vinyl ethers, olefin copolymers and mixtures thereof. The preferred pour point depressant is identified as polymethacrylate.
U.S. Pat. No. 6,495,495 teaches an additive comprising a blend of an alkyl ester copolymer, preferably an ethylene-vinyl acetate copolymer, and a naphthenic oil to improve flow properties of a mineral oil and to prevent filter blockage of a filter due to wax formation.
US 2006/0019841 teaches the use of a C12-C20 polyalkyl methacrylate polymer as a lubricating oil additive for mineral oil to improve the filterability of the lube oil as compared to the mineral oil base oil.
US 2003/0207775 teaches lubricating fluids of enhanced energy efficiency and durability comprising a high viscosity fluid blended with a lower viscosity fluid wherein the final blend has a viscosity index greater than or equal to 175. Preferably the high viscosity fluid comprises a polyalphaolefin and the lower viscosity fluid comprises a synthetic hydrocarbon or PAO and may further comprise the addition of one or more of an ester, mineral oil and/or hydroprocessed mineral oil. Additives can also be present and include one or more of dispersants, detergents, friction modifiers, traction improving additives, demulsifiers, defoamants, chromophores (dyes) and/or haze inhibitors.
The high viscosity fluid has a kinematic viscosity greater than or equal to 40 mm2/s @ 100° C. and less than or equal to 3,000 mm2/s @ 100° C. while the lower viscosity fluid has a kinematic viscosity of less than or equal to 40 mm2/s at 100° C. and greater than or equal to 1.5 mm2/s at 100° C. Haze inhibitors are not identified or described in any way.
It would be a significant technical advance if the haze issue associated with heavy GTL and hydrodewaxed or hydroisomerized waxy feed lube base stocks could be solved by a technique other than subjecting the base stock to an additional or more severe final processing step, such as more severe solvent or catalytic dewaxing or adsorption, or more severe hydrodewaxing or hydroisomerization all of which are marked by a reduction in yield.